1. Field of the Invention
The present invention relates to a motor drive circuit that can reduce fan noise.
2. Description of the Related Art
FIG. 1 is a block circuit diagram illustrating a conventional motor drive circuit.
Referring to FIG. 1, drive coils 1 and 2 are structurally attached in a fixed manner to the stator side of a motor. One end of each coil is grounded via a power source Vcc. A drive current IL1 and a drive current IL2 complementarily flow at every electrical angle of 180.degree.. A Hall element 3 is structurally attached at a predetermined position on the stator side of the motor and is connected between the power source Vcc and the ground to receive the power supply. The Hall element 3 produces a sine wave signal H+ and a sine wave signal H- of opposite phases, according to the relative position between the stator and the rotor. An amplifier 4 compares the sine wave signal H+ with the sine wave signal H- and then produces a rectangular wave signal. A control circuit 5 creates a rectangular wave signal B in an opposite phase based on the sine wave signal A output from the amplifier 4 and then current amplifies and outputs the rectangular wave signals A and B. The Darlington-connected NPN transistors 6 and 7 are connected between the other end of the drive coil 1 and the ground. When a high level rectangular wave signal A is applied to the base of an NPN transistor 6, NPN transistors 6 and 7 are turned on, thus causing a drive current IL1 to flow through the drive coil 1. Similarly, the Darlington-connected NPN transistors 6 and 7 are connected between the other end of the drive coil 2 and the ground. When the rectangular wave signal B of a high level is applied to the base of an NPN transistor 8, NPN transistors 8 and 9 are turned on and a drive current IL2 flows through the drive coil 2. Alternately, flowing the drive current IL1 through the coil 1 and the drive current IL2 through the coil 2 drives the motor.
FIG. 2 shows the waveforms of the drive current IL1 and the drive current IL2. The drive current IL1 (=IL) and the drive current IL2 (=IL) are expressed by the following formula; EQU IL=(VCC-Vsat-Ec)/RL
where VCC is a power source voltage; Vsat is a saturation voltage of the drive transistor 7 or 9; Ec is a counter electromotive force; and RL is a component resistance of the drive coil 1 or 2.
Since the counter electromotive force Ec becomes small at the phase changeover point T where the drive current IL1 or the drive current IL2 is switched during the motor running, the drive current IL2 becomes large. However, in actual conditions, the drive current IL at the phase changeover point IL does not efficiently produce the rotational torque because the abrupt change in magnitude of the drive current IL varies the rotational torque of the motor, thus inducing vibration noises of the motor.